The objective of this research is to provide a uniform set of thermochemical data on the interactions of the contractile proteins myosin and actin, together with head production measurements on the same species so that the various phases of heat production in muscle can be related to underlying biochemical steps of the cross bridge cycle. Toward this end we are making enthalpy measurements on actin interaction with myosin and myosin subfragments from frog, rabbit, and chicken muscles. Enthalpy measurements on myosin thick filament formation and actin polymerization are also planned. Heat measurements on frog muscle are being extended to include heat production following small quick releases of contracting muscle after the protocols of A. F. Hurley and R. Simmonds. This latter study is being done in collaboration with Dr. T. Blange of the Dept. of Physiology, University of Amsterdam. Finally, we are following a new lead revealed by our photon correlation studies that establishes the existence of rapid random fluctuating movements of individual myofibrillar sarcomers as a part of the mechanism of maintenance heat and shortening heat.